The overall goal of the proposed studies is to characterize further the intracerebral sites of action of gonadotropin releasing hormone (GnRH) and to identify the neurotransmitters which regulate the GnRH neuronal system in the rat. Specifically, we propose to: 1. Identify with immunohistochemistry and "in situ" hybridization the cells in brain which express the GnRH receptor. We recently generated a panel of specific antibodies against synthetic peptide sequences that are part of the intra- and extracellular domains of the pituitary GnRH receptor protein and we have developed cRNA probes to determine the sites of synthesis of the GnRH receptor in the brain. We propose now to determine: a. the location of the GnRH-receptor producing neurons; b. the identity of the neurotransmitters in the GnRH-receptor synthesizing neurons and whether the relative amounts of GnRH-receptor mRNA change in response to estradiol. 2. Identify the GnRH-mediated effects on hippocampal neurons. During the prior funding period we identified and characterized the structure/binding relationships of the brain GnRH receptors and showed that GnRH causes a transient induction of c-fos in certain hippocampal neurons. We propose now to determine whether GnRH causes the release of neuroactive messengers from the hippocampus and whether this effect is modulated by gonadal steroids. 3. Identify the second messenger systems which transmit the GnRH signal to the inside of the hippocampal target neurons. We will focus on the role of Ca++-influx and redistribution as well as on inositol phosphate hydrolysis since both messenger systems are activated by GnRH in the anterior pituitary gonadotropes. 4. Identify the norepinephrine neurons which mediate estradiol positive feedback actions. We propose: a. to determine precisely which noradrenergic cell groups project to the GnRH perikarya and to the GnRH axon terminals in the median eminence; b. to determine if these noradrenergic neurons contain steroid hormone receptors; c. to determine whether the gonadal steroids cause a change in the mRNA encoding for tyrosine hydroxylase. The proposed combination of morphological, physiological and molecular biological approaches will provide detailed information on the nature of the effects of GnRH in the hippocampus and will reveal some of the mechanisms by which the GnRH neuronal system is regulated. Together, the results will be important for our basic understanding of how the brain participates in the regulation of hormonal homeostasis and may provide clues for treatment of disorders caused by a central failure to maintain an adequate GnRH release.